1. Field of the Invention
This invention relates to a system for estimating the temperature of a vehicle hydraulically-operated transmission, more specifically to such a system for accurately estimating the temperature of a frictional engagement element such as a hydraulic clutch or some similar elements in a hydraulic actuation type automatic transmission which transmits drive torque by use of a hydraulic circuit and the frictional engagement elements.
2. Description of the Related Art
Generally speaking, vehicle automatic transmissions, particularly hydraulically-operated automatic transmissions, are operated by operating oil (automatic transmission fluid or ATF). The ATF hydraulic pressure is regulated in line with various objectives so as to control gear-shift shock, torque converter lockup, clutch slip and the like. Since the viscosity of the ATF varies with temperature (is high at low temperature and low at high temperature), the pressure rise and fall characteristics of the ATF also vary with temperature. This destabilizes the control.
Japanese Patent Laid-Open Application No. Sho 62(1987)-63,248 teaches a technology for coping with this problem by providing a solenoid valve in the hydraulic control circuit, controlling the duty ratio of pulse trains (Pulse Width Modulation) to be supplied to the solenoid valve to precisely regulate the oil pressure, providing an oil temperature sensor in the hydraulic control circuit, and correcting the duty ratio as a function of the detected ATF temperature, thereby preventing problems caused by variation in the ATF temperature.
Since oil temperature sensors are expensive, however, the general practice is to control the oil pressure of the automatic transmission using the engine coolant temperature, which is a required parameter for engine control anyway. Although the variation of the engine coolant temperature is ordinarily close to that of the ATF temperature, it may at times deviate greatly therefrom depending on the vehicle operating condition. Because of this, the prior art systems conduct the control using broadly defined temperature zones which cannot be used for fine control.
In Japanese Patent Laid-Open Application No. Hei 7(1995)-68,756 (filed in the United States under the number of Ser. No. 08/610,140 on Feb. 29, 1996), the assignee therefore proposed a control system for enabling accurate estimation of the ATF temperature from the engine coolant temperature or the like, without use of an expensive oil temperature sensor, thereby enabling fine hydraulic pressure control based on the estimated ATF temperature.
This earlier system can only estimate the average ATF temperature of the transmission as a whole, however, it cannot estimate the temperature rise produced momentarily by the large quantity of heat generated by the clutches during gear-shift. It is therefore susceptible to problems such as increased gear-shift shock caused by deviation of the coefficient of friction of the clutch friction material from the initial temperature characteristics and degradation of clutch durability owing to temperature increase.
An attempt to overcome these problems by equipping the clutches with oil temperature sensors or other such measurement means is problematic due to the fact that the clutches themselves rotate which would necessitate a complex sensor structure including electrical contacting means for taking out the sensor output signal, such as a slip ring and brush. Even then, high detection accuracy could not be expected.